1. Field of Invention
The present invention relates to a magnesium alloy suitably employable as materials of machine components to be used at high temperatures. Particularly, the invention relates to a heat resistant magnesium alloy appropriately employable as materials of engine components such as engine blocks (cylinder heads and cylinder block) and a transmission case of an automobile.
2. Description of Prior Art
Automobile industry has intended to use light-weight materials in place of iron and steel materials for manufacturing automobiles, in order to reduce the weight of the automobiles. As light-weight heat resistant alloys for engine components such as cylinder blocks and transmission cases which are machine components to be subjected to high temperatures, aluminum alloys (e.g., JIS ADC12 alloys) have been known.
Recently, the need of using light-weight materials for the engine components has further increased. Magnesium alloys have low specific gravity of about 1.8, which is less than that of the aluminum alloys (s.g.=approx. 2.7), and have various excellent characteristics. Therefore, the magnesium alloy are given much attention.
As magnesium alloys for materials of machine components, there have been known alloys of two different types, i.e., one type mainly containing aluminum (Al) (in the amount of about 4 to 10 weight %), and another type mainly containing Zn (in the amount of about 2 to 7 weight %, containing no aluminum). Some of such alloys are employed as heat resistant magnesium alloys for materials of machine components to be subjected to high temperatures. For examples, there have been known alloys such as ZE41A defined by ASTM and AE42 defined by DOW Standard.
The alloy ZE41A of ASTM is composed of 3.5 to 5.0 weight % zinc (Zn), 0.7 5 to 1.7 5 weight % rare earth metals (R.E.), 0.15 weight % or less manganese (Mn), 0.1 weight % or less copper (Cu), 0.01 weight % or less nickel (Ni), 0.3 weight % or less others and magnesium (Mg) of the remaining amount. The alloy AE42 of DOW Standard is composed of 3.5 to 4.5 weight % aluminium (A1), 2.0 to 3.0 weight % R.E., 0.27 weight % or less Mn, 0.20 weight % or less Zn, 0.04 weight % or less Cu, 0.004 weight % or less Ni, 0.004 weight % or less iron (Fe), 0.0004 to 0.001 weight % beryllium (Be), 0.01 weight % or less others and Mg of the remaining amount.
As R.E. (rare earth metals) incorporated into the above alloys, the misch-metal is generally employed. The representative composition of the misch-metal consists of 52 weight % cerium (Ce), 18 weight % neodymium (Nd), 5 weight % praseodymium (Pt), 1 weight % samarium (Sin) and 24 weight % lanthanum (La) and others.
The incorporation of R.E. is generally made to increase strength of the alloy at high temperatures. The R.E., however, is expensive so that the incorporation of R.E. into the alloy results in increase of cost for preparation of the magnesium alloy.
Further, in the case that the heat resistant magnesium alloys (ZE41A and AE42) containing R.E. is utilized for engine components such as engine blocks and transmission cases, the resultant components sometimes do not satisfy practical creep strength (minimum creep rate) and tensile strength at high temperatures which are required for the above engine components require.